1. Field of the Invention
The present invention relates to an image reading apparatus configured to read an image formed on an original, while conveying the original, an image reading unit thereof, and a reference member thereof for shading correction.
2. Description of the Related Art
In general, in an image reading apparatus configured to read an image formed on an original, while conveying the original, the original is illuminated with light from a light source, and reflected light from the original is focused by a rod lens array or the like, whereby the image on the original is read e.g. by a line image sensor. However, due to variations in the amounts of light from the light source and the variations in the performance of the rod lens array and variations in the sensitivity of the line image sensor, it is impossible to perform uniform image reading without any correction.
To enable uniform image reading, image data obtained from an output signal from the line image sensor when an original image is read is corrected using shading correction data formed based on an output signal from the line image sensor obtained when a white reference member is read. When correction data for use in the correction is captured, light amount adjustment for optimizing the amount of light emitted from the light source to illuminate an original, and gain adjustment for optimizing an amplification factor for amplifying image signal output from the line image sensor are carried out. Further, it is a general practice to perform shading correction for correcting the variations in the amounts of light from the light source and the variations in the performance of the rod lens array and the variations in the sensitivity of the line image sensor in association with each pixel thereof. Hereafter, the correction including the light amount adjustment and the gain adjustment performed so as to enable the line image sensor to uniformly read image information from the original will be referred to as “shading correction”.
In an image reading apparatus of the above-mentioned type, a member disposed in an original image reading position where the line image sensor reads an image on an original, for supporting a conveyed original from the reverse side of the same original has a color (black in general) other than white for the purpose of detection of a boundary between the original image and the background image, detection of a skew-feeding of the original, and prevention of lack of hiding of a reverse side image of the original.
A description will be given of the schematic arrangement and operation of a conventional typical image reading apparatus with reference to FIG. 22.
The image reading apparatus 1000 shown in FIG. 22 is configured to read image information on an original D by a line image sensor 110 through a contact glass 150 while conveying the original D. The image reading apparatus 1000 includes a pickup roller 2 for picking up originals D, a feed roller 3 for feeding the originals D picked up by the pickup roller 2, a retard roller 4 for separating the picked-up originals D one from another, and a registration roller pair 5 and a convey roller pair 7 each formed by a pair of rollers disposed at respective opposed locations, for conveying the originals D.
First, before reading an image on an original D, the image reading apparatus 1000 moves the line image sensor 110 in a direction indicated by an arrow S in FIG. 18 and causes the line image sensor 110 to read a reference member 170. The reference member 170 is disposed at a location opposed to the line image sensor 110 having been moved in the direction indicated by the arrow S.
Then, image data obtained based on an output from the line image sensor 110 when the line image sensor 110 has read the reference member 170 is stored, as shading correction data for shading correction, in association with each pixel of the line image sensor 110. Thereafter, the image reading apparatus 1000 returns the line image sensor 110 to its original position (original image reading position), and causes the line image sensor 110 to read the image on the original D, while conveying the original D. During the operation for reading the original D, the image data obtained from the output of the line image sensor 110 is corrected by shading correction performed with reference to the correction data stored in advance. It should be noted that whether or not the line image sensor 110 has been moved to a reference member reading position for reading the reference member 170 can be determined based on an output from a position sensor 120. The position sensor 120 is a means for detecting the position of the line image sensor 110.
In the image reading apparatus 1000 configured as above, if the line image sensor 110 deviates from the exact original image reading position when the line image sensor 110 returns from the reference member reading position to the original image reading position, registration error occurs in original reading. This registration error is generally prevented by positioning the line image sensor 110 using the position sensor 120. The above-described sequential operation is commonly performed in response to an instruction from a control means, such as a CPU (Control Processing Unit).
Some image reading apparatuses are configured such that not a line image sensor but a reference member is moved between original reading position and escape position. For example, a technique has been proposed in which the reference member is exposed into an original conveying path during a period of reference member reading, and is retracted to a position where the reference member does not contact with a conveyed original, during a period of original reading (see e.g. Japanese Laid-Open Patent Publication (Kokai) No. 2005-102017).
Conventionally, the reference member is disposed on the far side of the original conveying path from the line image sensor, at a location which enables the line image sensor to read the reference member.
However, in the above-described prior art, paper powder and the like dust comes to stay between the reference member and the line image sensor as originals are conveyed, to make the reference member soiled, which can often hinder accurate shading correction. In such a case, a shading mechanism originally configured to correct the variations in the sensitivity of the line image sensor and the variations in the amount of light emitted from the light source and the variations in the performance of the lens array cannot operate accurately, causing variations in color and brightness in a read image. This is a serious problem for an image forming apparatus.
For this reason, when accurate shading correction cannot be performed on images due to the above-mentioned soiling, a first countermeasure is conventionally taken in which a user cleans the reference member, and then reading is resumed.
Further, a second countermeasure has conventionally been proposed in which the reference member is disposed at a location which does not face the original conveying path and where dust and dirt are difficult to attach, and the reference member is moved to a reading position of the line image sensor when acquisition of shading correction data is performed using the line image sensor (see e.g. Japanese Laid-Open Patent Publications (Kokai) No. H10-304195 and No. 2005-102017).
A conventional image reading apparatus of another type has an image reading unit which causes the line image sensor 110 to slidably move in a direction parallel to the contact glass 150 thereby enable the reading position of the image sensor to move between the original reading position and the reference member reading position. This type of image reading apparatus reads image information from an original D by the line image sensor 110 through the contact glass 150 while conveying the original D in a direction indicated by an arrow ‘a’ in FIG. 23. During this operation, a light source incorporated in the line image sensor 110 illuminate the original D through the contact glass 150. First, before starting the reading of the original D, the image reading apparatus moves the line image sensor 110 in a direction indicated by an arrow b in FIG. 23, and causes the same to read a surface (reference surface), which faces the contact glass 150, of the reference member 170. Then, the image reading apparatus generates shading correction data for use in shading correction, based on a signal output from the line image sensor 110 having read the reference surface, and stores the generated shading correction data on a pixel-by-pixel basis.
Thereafter, the image reading apparatus returns the line image sensor 110 to its original position to read the image on the original D while conveying the original D in the direction of the arrow a. During this image reading operation, the image reading apparatus performs shading correction on image data generated based on an output signal from the line image sensor 110, by referring to the shading correction data stored in advance.
The reference member 170 used here is implemented e.g. by a white film formed by silk printing or the like.
Japanese Laid-Open Patent Publication (Kokai) No. S62-098861 discloses a flat bed scanner which uses the aforementioned white film as the reference member.
The image reading apparatus disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2005-102017 is advantageous in that originals do not rub against the reference member during conveyance, and hence the reference member is difficult to be scratched or soiled. However, paper powder produced and wafted during an original conveying period can be deposited on the reference member, which necessitates periodic cleaning of the reference member. In addition, since paper powder is very fine, it is difficult to remove the paper powder deposited on the reference member completely. Further, a mechanism for causing the reference member to be exposed to the original conveying path is necessitated, which can cause an increase in the number of component parts and make the construction of the apparatus complicated.
On the other hand, in the conventional image reading apparatus shown in FIG. 22, since an original D rubs against the reference member 170 during conveyance, the reference member 170 can be scratched, or dust, such as paper powder and swarf from rollers, can be attached to the reference member 170. Therefore, if shading correction is performed with reference to the reference member 170 deprived of whiteness by being scratched or soiled, lines or streaks appear on read original images.
Insofar as the first countermeasure is concerned, image degradation due to maladjustment of shading correction cannot be recognized until the read image is viewed. For this reason, in a case where a large number of originals are continuously read, it is difficult to recognize image degradation at an early stage of the reading operation, and it is only after a long time that the reading operation is restarted. Further, the user has to clean the reference member manually, which is a nuisance to the user.
On the other hand, in the case of the second countermeasure, a traveling path along which the reference member extends continuous with the original conveying path and a standby location of the reference member, and hence a tiny amount of dust, such as paper powder, produced by conveyance of originals can enter the traveling path and stay thereon. In this case, there is a fear that the reference member moved into the original conveying path for shading correction might be soiled by the remaining dust.
Further, white films formed by coating (painting or printing) are liable to differ in thickness between individual units thereof, and even an individual white film is apt to have variation in thickness depending on the location thereon. The use of such a white film as a white reference leads to a single image which is partially degraded due to partial degradation of the image reading performance or occurrence of an image reading apparatus degraded in image reading performance.